1. Field of the Invention
The present invention relates to a sound absorbing device including a Helmholtz resonator, and an electronic device and an image forming apparatus including the sound absorbing device.
2. Description of the Related Art
In an electrophotographic image forming apparatus, there are generated the driving sound of various driving units, the rotating sound of a polygon mirror, and the like. As a configuration that can absorb the sound generated in the image forming process, Japanese Patent Application Laid-open Nos. 2000-235396 and 2001-117451 disclose an image forming apparatus including a sound absorbing device having a Helmholtz resonator.
The Helmholtz resonator is formed of a cavity part having certain capacity and a communicating part that causes the cavity part to communicate with the outside. Assuming that a volume of the cavity part is “V”, an square measure of the communicating part is “S”, a length of the communicating part in a communicating direction is “H”, and the velocity of sound is “c”, a frequency “f” of sound absorbed by the sound absorbing device including the Helmholtz resonator is obtained through the following expression (1).
                    f        =                              c                          2              ⁢                                                          ⁢              π                                ⁢                                    S                              V                ⁡                                  (                                      H                    +                                          Δ                      ⁢                                                                                          ⁢                      r                                                        )                                                                                        (        1        )            (Δr: open end correction)
The Helmholtz resonator can absorb sound that should be prevented from being transmitted to the outside of the apparatus by setting the volume V of the cavity part, the square measure S of the communicating part, and the length H of the communicating part corresponding to the frequency of the sound that should be prevented from being transmitted to the outside of the apparatus based on the expression (1).
However, when an air current is generated around an opening of the communicating part that causes the cavity part of the Helmholtz resonator to communicate with the outside, resonance is hindered and a sound absorbing effect of the sound absorbing device including the Helmholtz resonator may be unfortunately reduced in some cases.
In view of the above-mentioned conventional problem, there is a need to provide a sound absorbing device including the Helmholtz resonator to prevents reduction in the sound absorbing effect due to the air current around the opening and efficiently absorb the sound, and the electronic device and the image forming apparatus including the sound absorbing device.